Cobalt and rare earth metals are known as valuable metals, and have various uses in industry. Cobalt is used, for example, in positive electrode materials for secondary batteries, and further for superalloys (high strength heat resistant alloys), which are used in jet engines of aircraft, for example. Rare earth metals are used in phosphor materials, negative electrode materials for nickel-hydrogen batteries, additives for magnets built into motors, abrasives for glass substrates used for liquid crystal panels and hard drives, and the like.
In recent years, energy conservation has been strongly promoted, and the changeover from conventional gasoline-fueled cars to hybrid cars and electric cars with a secondary battery using cobalt and rare earth metals is being rapidly made in the automotive industry. Also, the changeover from conventional fluorescent tubes to efficient three-wavelength fluorescent tubes using rare earth metals such as lanthanum, cerium, yttrium, terbium and europium is being rapidly made in lighting equipment. The above cobalt and rare earth metals are scarce resources, and most of them are obtained via imports.
However, yttrium and europium are used in the phosphors in CRT-televisions for analog broadcasting, and a large number of cathode-ray tubes have been discarded as used products with the changeover to liquid crystal television in recent years. It can be easily expected that products such as secondary batteries and three-wavelength fluorescent tubes, which have rapidly spread, will also represent a large amount of waste in the future as used products. As described above, it is not preferred that cobalt and rare earth metals, scarce resources, not be recycled from used products and be treated as waste from the viewpoint of resource conservation and resource security. The establishment of a method for effectively recovering valuable metals such as cobalt and rare earth metals from such used product has been strongly demanded recently.
<Recovery of Cobalt from Secondary Battery>
Examples of the above secondary batteries include nickel-hydrogen batteries and lithium-ion batteries and the like, and cobalt and further manganese, rare metals, are used in the positive electrode materials therefor. In positive electrode materials for lithium-ion batteries, the percentage of low-priced manganese tends to be raised in the place of high-priced cobalt. Recently, the recovery of valuable metals from used batteries has been attempted, and one recovery method is a dry method in which used batteries are put into a furnace and dissolved to separate into metals and slag and the metals are recovered. In this method, however, manganese moves to the slag, and thus only cobalt is recovered.
In addition, a wet method in which used batteries are dissolved in an acid and metals are recovered using a separation method such as a precipitation method, a solvent extraction method or an electrowinning method, is also known. In the precipitation method, for example, a method in which the pH of a solution containing cobalt and manganese is adjusted and a sulfurizing agent is added thereto to obtain precipitates of sulfurized cobalt, and a method in which an oxidizing agent is added to obtain precipitates of oxidized manganese are known (e.g., see Patent Document 1). In this method, however, there are problems in that, for example, coprecipitation occurs, and it is difficult to completely separate cobalt and manganese.
When attempts are made to recover cobalt as a metal by the electrowinning method, it is known that manganese oxides are precipitated on the surface of the anode in a system in which a high concentration of manganese exists, and the deterioration of the anode is promoted. In addition, fine manganese oxides with a specific color float in an electrolytic solution, which, for example, causes clogging of the filter cloth used in electrowinning and the contamination of the cobalt metal with the manganese oxides, and thus stable operations are difficult.
Acid extractant is widely used when attempting to recover cobalt using the solvent extraction method. As described above, however, because a large amount of manganese is used in positive electrode materials for lithium-ion batteries recently, a high concentration of manganese exists in the solution of batteries. There are no effective extractants to selectively and effectively extract cobalt from such a system.
A nickel ore, such as nickel oxide ore is used as a raw material in the recycling of used batteries and further cobalt smelting. The ratio of manganese is higher than that of cobalt in nickel oxide ore, and the existing ratio thereof is about 5 to 10 times that of cobalt, and separation from manganese is a large problem in cobalt smelting.
<Recovery of Rare Earth Metal from Three-Wavelength Fluorescent Tubes and Cathode-Ray Tubes>
A mixture of rare earth metals such as lanthanum, cerium, yttrium, terbium and europium is used in the phosphors used in the three-wavelength fluorescent tubes mentioned above. Furthermore, yttrium and europium together with a high ratio of zinc are contained and used in phosphors for cathode-ray tubes.
As a method for recovering a specific rare earth metal from a mixture of rare earth metals, a method for recovering the specific rare earth metal from a liquid, in which the rare earth metals are dissolved in an acid such as mineral acid, by the solvent extraction method is generally used. An industrial example using the trade name PC88A (manufactured by DAIHACHI CHEMICAL INDUSTRY CO., LTD.), a phosphorus-based extractant exists for the mutual separation of rare earth metals, for example. However, because this extractant contains phosphorus in the structure, an advanced wastewater treatment system is required to prevent the extractant and deteriorated materials thereof in discharged water polluting public water areas when industrially using the extractant. The extractant is subject to total volume control in accordance with the Water Pollution Law on specific areas in Japan, and thus there are concerns when using the extractant on an industrial scale.
As a phosphorus-free extractant, a carboxylic acid-based extractant (e.g., 2-methyl-2-ethyl-1-heptanoic acid: neodecanoic acid) is put to practical use. However, this extractant acts in extraction only in a high pH range, neutral or higher, and thus when an acid solution as described above is used, a large amount of neutralizer is required, and so cost increases are a concern. Furthermore, the extraction ability of the carboxylic acid-based extractant is lower than that of the phosphorus-based extractant described above, and excessive equipment is required, and thus there is also a problem of cost increases.
To solve such problems, an extractant called DODGAA having the skeleton of diglycol amic acid is developed (e.g., see Patent Document 2). However, when this extractant is used, as shown in Non-Patent Document 1, yttrium (Y), lutetium (Lu), ytterbium (Yb), thulium (Tm), erbium (Er), and holmium (Ho), called heavy rare earth metals among rare earth metals, have a strong tendency to be extracted with dysprosium (Dy), terbium (Tb), gadolinium (Gd), europium (Eu), and samarium (Sm) called middle rare earth metals, and thus the extractant is not suitable for mutual separation of rare earth metals. In the case of DODGAA, the extraction rate of promethium (Pm), neodymium (Nd), praseodymium (Pr), cerium (Ce), and lanthanum (La), called light rare earth metals, is also low. Europium (Eu), which is produced in especially small amounts and expensive, cannot be selectively recovered from other rare earth metals. As described above, an extractant by which the mutual separation of rare earth metals is possible, and further an extractant which can efficiently extract light rare earth metals have not yet been found.
To solve the problems, it is suggested that light rare earths, which have been difficult to separate in Patent Document 2 mentioned above, are efficiently separated using a specific amide derivative (e.g., see Patent Documents 3 and 4). When this amide derivative is used, outstanding characteristics, which conventional extractants do not have, are obtained, for example a small amount of cobalt can be extracted from a high concentration of manganese. Furthermore, the above amide derivative also has a feature to be able to specifically extract scandium among rare earth elements, and scandium is known to exhibit behaviors different from those of other rare earth elements. The amide derivative is thus suitable, for example, for recovering a small amount of scandium contained in nickel oxide ore from a solution obtained by acid leaching of the nickel oxide ore.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2000-234130    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2007-327085    Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2013-216966    Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2013-189675    Non-Patent Document 1: K. Shimojo, H. Naganawa, J. Noro, F. Kubota and M. Goto; Extraction behavior and separation of lanthanides with a diglycol amic acid derivative and a nitrogen-donor ligand; Anal. Sci., 23, 1427-30, 2007 December.
For example, in the case of the above acid solution obtained by acid leaching of nickel oxide ore, the concentration of scandium contained in the acid solution is about several to several tens mg/1, which is extremely low. When the above amide derivative is used for solvent extraction treatment, because the concentration of scandium contained in the acid solution is extremely low, a large amount of extractant is required for the solvent extraction treatment. Furthermore, due to the large amount of extractant, the equipment sizes, extraction tank and liquid storage tank for example, are also expanded on a similar scale, and thus the increase in spending on the equipment necessary becomes a problem.
In general, in the case of the solvent extraction treatment, when the conditions such as the mixing ratio of extractant and acid solution and liquid temperature are not constantly maintained, extraction characteristics are changed, and stable operations tend to be difficult to accomplish. Therefore, detailed operation control is required. In addition, when air is caught during extraction operation, inclusion called CRUD is produced by oxidization of iron ions contained in the solution, and this involves solvent extraction operations being inhibited. In particular, a high concentration, several g/l or more, of divalent iron ion is contained in the above acid solution obtained by acid leaching of nickel oxide ore in many cases, and thus it is preferred that the acid solution be used for a solvent extraction step after the concentration of divalent iron ions contained therein is kept as low as possible.
As described above, it is required that a large amount of raw material be treated and detailed operation control be carried out in this treatment, when, for example, recovering a trace metal from nickel oxide ore.